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The viscosity () dependence of the folding rates for four sequences (the native state of three sequences is a sheet, while the fourth forms an helix) is calculated for off-lattice models of proteins. Assuming that the dynamics is given by the Langevin equation, we show that the folding rates increase linearly at low viscosities, decrease as 1/ at large, and have a maximum at intermediate values. The Kramers' theory of barrier crossing provides a quantitative fit of the numerical results. By mapping the simulation results to real proteins we estimate that for optimized sequences the time scale for forming a four turn -helix topology is about 500 ns, whereas for sheet it is about 10.
Klimov et al. (Mon,) studied this question.